Specialize `PutBackN::fold` #640

Job	Run time
test	51s
check format	7s
check	6s
msrv	27s
check (--no-default-features)	4s
check (--no-default-features --features use_alloc)	5s
check (--all-targets --all-features)	22s
All checks succeeded	1s
	2m 3s

Provide feedback